Which of the following is a secondary messenger?

A secondary messenger is a molecule that relays signals received at receptor cell surfaces to target molecules inside the cell, typically amplifying the strength of the signal. cAMP, or cyclic adenosine monophosphate, is a well-known secondary messenger involved in various biological processes. It is synthesized from ATP in response to hormonal signals and activates specific protein kinases, thereby leading to a physiological response within the cell.

In contrast, DNA is a genetic material that carries the instructions for the development and functioning of living organisms and does not function as a signaling molecule. Protein kinases are enzymes that facilitate the transfer of phosphate groups from ATP to target proteins, playing a crucial role in signal transduction but not acting as secondary messengers themselves. ATP, while essential as an energy currency in cells, is not classified as a secondary messenger, even though it can be a precursor for cAMP. Therefore, cAMP stands out as the correct answer for its role in transmitting signals within the cell.